Iron pyrite (FeS2) is a promising photovoltaic material because of its strong light absorption (α>105 cm−1 for hv>1.3-1.4 eV), sufficient minority carrier diffusion length (100-1000 nm), and essentially infinite elemental abundance. Pyrite photoelectrochemical and solid-state Schottky solar cells have shown large short-circuit current densities (30-42 mA cm−2) and quantum efficiencies (up to 90%). However, the band gap of pyrite (Eg=0.95 eV) is somewhat narrow for optimal photovoltaic applications according to the Schockly-Queisser theory and, more seriously, the open-circuit voltage of pyrite-based devices is too low (Voc≈0.2 eV, only ˜20% of Eg). It should be easier to achieve a larger Voc by starting from a larger bulk band gap. Therefore, identifying practical ways to increase the band gap of pyrite is of interest for the development of pyrite-based photovoltaics.
Substituting cations or anions with isovalent elements or compensated dimers is a widely-used approach to modify the band gaps of other semiconductors, including III-Vs and ternary ABC2 chalcopyrite compounds. A recent density functional theory (DFT) study by Sun et al. found that the band gap of pyrite can be increased slightly by replacing some Fe by Ru or Os to form Fe1−xRuxS2 and Fe1−xOSxS2 compounds. However, this approach is limited by the overall weakness of the effect (Eg increases by only ˜0.1 eV even at x=0.5) and the low solubility of Ru and Os in pyrite.
Upon investigation of Fe1−xZnxS2 alloys, it was determined that substitutional zinc (ZnFe) results in significant band gap narrowing for x up to 0.5 (i.e., 50% ZnFe), in spite of the large band gap of pyrite ZnS2 itself (˜2.5 eV). The calculations on Fe1−xZnxS2 alloys are in good agreement with the results of Sun et al. Substitution of Zn for Fe produces gap states and new valence bands above the Fe-hg bands of pyrite, leading to a narrowed band gap at large ZnFe concentrations. The unexpected ineffectiveness of cation alloying calls for a new approach to increase the band gap of iron pyrite.
Accordingly, improved methods for increasing the band gap of pyrite, thereby improving the overall band gap and open circuit voltage of pyrite-based devices are needed.